The present invention generally relates to implantable prostheses and the like and to methods for making same. More particularly, the invention relates to a graft, such as a vascular graft or AV-shunt, having a compliant porous inner component and a compliant porous load-bearing outer component, bound together by a porous intermediate component that is made of material having a melting point lower than that of the materials from which the inner and outer components are made. With the outer component bound by the intermediate component to the inner component, a porous, yet strengthened integral graft results.
Blood vessels are not straight, rigid tubes but elastic conduits made of a variety of materials and having a compliance that varies with functional considerations. For example, the venous system functions, in part, as the blood reservoir for the body. In order to be able to respond to a larger volume of blood sent into the system because of, for example, a change in arterial blood pressure, the vessels of the venous system must be sufficiently compliant so that they can distend. The arterial system functions as the body's pressure reservoir. In order to avoid the wide swings in the blood pressure and flow that are possible with every contraction and relaxation of the heart, yet be able to maintain sufficient blood pressure so that blood can be pushed into all regions of the body, including through the small-diameter arterioles and the microcirculatory bed, the arteries must have sufficient compliant strength to elastically expand and recoil without the marked distension of the venous system.
Conventional grafts, however, are generally made of materials and in shapes that provide a structure whose compliance is markedly different from that of the walls of the vessel to which they may be attached. Grafts having walls less compliant than that of the host vessel walls are problematic in that conditions, such as intimal hyperplasia and stenotic narrowing, may develop. Grafts with walls having greater compliance than that of the vessel to which the graft is attached are problematic in that a portion of the graft wall may balloon--that is, develop an aneurysm--after implantation.
Other known grafts, while they may be compliant, may not necessarily be made from biocompatible materials. The implantation of a graft made from such material may prompt a thrombogenic or immunological response with the resultant deleterious formation of microthrombi or microocclusions in and around the graft. Other grafts are made from generally non-porous materials, that, accordingly, do not facilitate the ingrowth of cells and tissue within the graft. The full incorporation of the graft into the surrounding host tissue is thereby frustrated. Still other conventional grafts are made from microporous textiles that require preclotting of the vessel wall with blood to prevent leakage of blood at implantation.
A demand therefore is present for an integral graft made from biocompatible materials and having a structure that has compliant strength similar to that of natural tissue but that is sufficiently porous so that the graft may become incorporated into the host tissue yet not leak blood. The present invention satisfies the demand.
The present invention includes a three component system, an inner component, an intermediate component, and an outer component. While the components may be made from materials having generally different melting points and different mechanical properties, at a minimum the inner component and outer component are made from a material or materials having a melting temperature higher than the material from which the intermediate component is made. More specifically, the inner component is porous and is made from a biocompatible synthetic material, preferably a polyurethane composition made with an aromatic polycarbonate intermediate, having a melting point that is, at a minimum, in excess of the melting point of the composition from which the intermediate component is formed (further discussed below).
There are many methods by which the inner component may be made, such as the many known methods used to produce porous compliant vascular prostheses. One such method is termed phase inversion or separation which involves dissolving a urethane in a solvent, such as dimethyl acetamide (DMA), forming a coat on a mandrel--such as by dipping the mandrel into the dissolved urethane--and then immersing the urethane coating in a solution such as water by which DMA may be dissolved, but not urethane, thereby causing the urethane to bead-up and form a porous matrix.
Another method by which the inner component may be formed is termed particle elution. The method utilizes water soluble particles such as salt (NaC1, MgC12, CaCo2, etc.) polymers, such as polyvinylpyrrolidone, sugars etc. The particles are mixed or blended into a urethane composition, and after forming a graft from the mixture such as by dip coating or extruding the particle filled plastic, the particle is eluted out with a suitable solvent.
Additional methods include replamineform, that involves the dissolution of a matrix, such as that of a sea urchin, out of the urethane with hydrochloric acid, spray techniques where filaments or beads of urethane are sprayed onto a mandrel to produce a porous vascular graft, and electrostatic deposition of urethane fibers from solution.
However, the porous vascular graft preferred in this invention is prepared according to the method detailed in U.S. Pat. No. 4,475,972 to Wong. This patent is incorporated hereinto by reference. An antioxidant may be added to further prevent degradation of the fibers drawn of the material from which the inner component is made.
Regardless of the nature and method of manufacturing the porous inner component, the intermediate component is comprised of one or more layers of a biocompatible synthetic material, preferably a polyurethane material, having a melting point lower than the melting point of the material from which the inner component is formed and lower than the melting point of the material from which the outer component is made.
The outer component comprises a mesh network made of strands, fibers, beads or expanded versions of a durable material such as a composition of fluorocarbons, such as expanded polytetrafluoroethylene ("ePTFE")--commonly termed Teflon--or stable polyesters, such as preferably polyethylene terephthalate ("PET")--commonly termed Dacron. This material is preferably warp-knitted in a tricot or double tricot pattern and shaped in a tubular configuration. It can also be appreciated that the outer component can be woven, braided, weft-knitted and the like with loose fibers, textured fibers and the like to provide increased compliance. With the three components in place, a composite graft according to the present invention is formed by heating the structure to a temperature at or above the melting point of the material from which the intermediate component is formed but below the melting temperature or temperatures of the material from which the outer component is formed and of the material from which the inner component is formed. In this temperature range, the intermediate component may melt without the melting of either the inner component and the outer component, thereby mechanically bonding the inner component to the outer component.
The multi-component system of the present invention provides a number of advantages over conventional grafts. The use of a durable material, such as PET or ePTFE, from which the outer component may be formed is advantageous because of the known strength that such material has in the body. Devices made from PET or ePTFE when implanted in the body are known to maintain their integrity for some three decades. Further advantageously, it has been found that a graft--made according to the present invention and in which PET is used to form the outer component --has a burst strength and a tensile strength that is some two times greater than that of a conventional graft. Such strength prevents the dilation of the vessel in response to, for example, an increase in blood flow and/or pressure, creep relaxation of the urethane, biodegradation of the urethane, plasticization of the urethane, etc. Decreases in the strength of PET that may occur after implantation due, for example, to the absorption of water after implantation, are minimal as Dacron has a low water absorption ability.
The use of a knitted pattern according to which the durable strands of the outer component may be configured is advantageous due to the increased compliance such a pattern provides. As stated above, a durable material such as PET is recognized as a strong yet not necessarily compliant material. However, by knitting the strands from which the outer component is formed into a network, a compliant reinforcing outer component is formed. The use of such a material from which to form the outer component in the three component system of the present invention advantageously provides a strengthened, yet compliant graft.
The winding of strands of synthetic material, such as polyurethane over a mandrel to form an inner component is further advantageous because of the resultant porosity of the component. While the intermediate component may be made porous, for example, by painting synthetic material over the inner component and utilizing the phase inversion method or the particle elution method to form a porous matrix, preferably the intermediate component is formed by winding strands of synthetic material, such as polyurethane over the inner component, to provide a highly porous network. Utilizing strands of PET configured in a knitted pattern to form the outer reinforcement component further provides a porous network. Advantageously, by combining these individually porous components together in a composite graft, a totally porous integral graft results. Porosity is an advantage in medical devices, such as vascular grafts, because an open structure allows vascular fluid to infiltrate and communicate to and from the surrounding tissue and the interior of the graft and allows the ingrowth of tissue to occur within the graft. Accordingly, the device becomes better incorporated into the surrounding tissue, thereby further securing the device within the implantation site.
Uniting the three components into a single composite graft advantageously facilitates the use of the device. The graft may be implanted without the need for any assembly immediately prior to use. The graft may be also cut and/or sutured as a unit without the need for the separate cutting and/or suturing of each component. Methods for cutting the composite graft include scalpel, scissors, hot wires, shaped blades, and the like. The speed with which the graft may be implanted is a particularly distinct advantage since the device is implanted only when a patient is undergoing surgery.
The use of a polycarbonate intermediate rather than, for example, a polyether urethane to make the polyurethane material from which the inner component is preferably made is advantageous as the resultant inner component better resists degradation. The resistance to degradation is further aided by the addition of antioxidant to the material from which the inner component is formed.
It is, accordingly, a general object of the present invention to provide an improved graft.
Another object of the present invention is to provide an integral improved graft made from a composite of layers of synthetic materials.
It is also an object of the present invention to provide a graft that is totally porous thereby facilitating the incorporation of the graft into the site of implantation.
An additional object of the present invention is to provide an improved graft having an outer component which strengthens the device without significantly impairing the overall compliance of the graft.
These and other objects, features and advantages of this invention will be clearly understood and explained with reference to the accompanying drawings and through a consideration of the following detailed description .